Troubleshooting An Alternator Warning Light

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We interrupt this series on the Decapitation of Otto to bring you something less greasy and possibly more useful. Last week I was preparing to attend the Nor’East 02ers’ Spring Drive. I was going to take Otto, but he was, well, you know, decapitated. So I prepared to take Kugel, my ’72 2002tii, out of storage. This made sense, since I plan to drive Kugel to the Vintage next month, and I thought that a check-out after the winter slumber might be in order.

I didn’t know how right I was.

Kugel was stored for the winter out in Fitchburg, 40 miles away. I drove out in the E39 with a freshly-charged battery in the trunk. I dropped the battery into Kugel, and with some coaxing, the car started up.

One of the basic electrical checkups you can do in a car is to use a multimeter to measure the resting voltage—the voltage across the battery terminals with the car off. It should be about 12.6 volts. (Yes, 12.6, not 12, because a standard 12V car battery actually consists of six individual 2.1-volt cells.) Then you start the engine and measure again. When the engine is running and the alternator is charging the battery, the voltage across the battery should measure about a volt or two higher—that is, it should read about 13.5 to about 14.5 volts. If instead it’s still showing 12.6V, it means that the alternator isn’t charging the battery, and even if the car is running now, at some point down the road the car will die, because the battery alone can’t satisfy the electrical demands of the car for very long.

Instead of checking these voltages with a multimeter, you can spend seven bucks online and buy a little digital voltmeter that plugs into the cigarette lighter. These things are great; everyone should have one in the glovebox of every car they own. That way, if a car won’t start due to a dead battery, after the car has been jump-started, you can tell whether the alternator is recharging the battery. If it is, you’re probably good to go. (Well, maybe. If it’s a post-1990-ish computer-laden car, it may still do weird things until it has a good battery in it.) But if the cigarette-lighter voltmeter isn’t reading 13.5 to 14.5 volts and is instead reading closer to battery voltage, the car is virtually guaranteed to die a few miles down the road.

(If I may indulge in a blatantly self-serving plug, I discuss this all in detail in my new book, The Hack Mechanic Guide to European Automotive Electrical Systems; see the link at the bottom of this post.)

So anyway, as I was about to drive home the 40 miles from Fitchburg, I plugged my portable voltmeter into Kugel’s lighter socket. Imagine my surprise when I looked at it and saw not 13.5 to 14.5V, not 12.6V, but only 9.2V. Wait, WHAT?

I had a general-purpose multimeter with me, and I used that to double-check the voltage in order to make sure that the little cigarette-lighter voltmeter hadn’t gone wonky. The multimeter confirmed the reading. So not only wasn’t the alternator charging the battery, the “fully charged battery” I’d dropped into the car was toast.

It dawned on me that if I didn’t put Kugel back in the garage RIGHT NOW, I’d have a dead tii out on the street in Fitchburg. Back in the garage it went.

So much for driving the tii to the Nor’East 02ers’ Spring Drive. I took the 635CSi instead, a move that proved to be the seed that bloomed into my decision to drive it to Sharkfest, an adventure I will cover in Roundel Magazine.,

Meanwhile, as I pondered the tii’s non-functional alternator, I realized that I hadn’t checked to see if the alternator light was on. That is, if an alternator isn’t charging the battery and the alternator light is on, the problem is likely to be the alternator—but if the alternator light doesn’t come on at all when you turn the key, there is a problem with the light itself that has to be resolved first. (This is also discussed in detail in my new book. Which, as you can see after this column text, is promoted in the aftertext. Unless Satch slices it out in a fit of jealousy because his book has been out of print for twenty years, and now I have two.)

Some people have heard that on “old” BMWs with a four-wire alternator and an external voltage regulator—meaning 2002, Bavaria, and E9-era cars—as well as on cars with a two-wire alternator—meaning, mostly, pre-1996 cars—if the alternator light is burned out, the alternator won’t work because it’s wired in series with the light. People understandably think What idiot would design a charging system like a 30-year-old string of Christmas-tree lights so it won’t charge if the warning light is out?

It’s a little more complicated than that. Here’s how it works: When you crack the key to the ignition setting, one of the alternator light’s two wires is supplied with battery voltage. The light’s other wire is connected to the alternator’s D+ terminal. On a two-wire alternator, that’s usually a small threaded post, although it may be a quick-disconnect terminal. On a four-wire alternator, it’s one of the wires (the blue one) on the connector to the external voltage regulator.

In either case, the D+ wire supplies the alternator with what’s known as Excitation Current. This is the mechanism that bootstraps the alternator to do its thing of converting a varying magnetic field into an electric current.

So the alternator warning light is doing two things: It provides the excitation current to kick the alternator into functionality, AND it acts as a warning light by glowing when there’s more than a two-volt difference across the light’s two wires.

Initially, when you crack the ignition key, but before the alternator spins up, the D+ line is grounded through the alternator itself, so with 12V on one wire of the light and the other wire grounded, 12V flows through the light to ground. This creates more than a 2V difference across the light, so it lights up. But as the alternator spins up and generates about 14V into the D+ line, the difference between D+ and battery voltage becomes less than 2V, so the light goes out.

On an old four-wire alternator with an external regulator, the wiring looks like this:

[Illustration courtesy Bentley Publishers all rights reserved]

If the alternator has two wires—a fat B+ wire from the battery and a small blue D+ exciter wire—the wiring of the warning light looks like this:

[Illustration courtesy Bentley Publishers all rights reserved]

Now that you know how the light works, the best part is that it’s very easy to test whether an alternator’s non-functionality is simply due to the light. If you crack the key to ignition and the alternator light doesn’t come on at all, the problem may simply be the bulb. Pull the instrument cluster out and swap the bulbs—for example, take the functioning oil-pressure-light bulb (your oil-pressure light IS working, right?) and swap it with the alternator light bulb. If the alternator light then goes on when the key is cracked, the problem was just a bad bulb. (Don’t forget to replace the bulb, which is now useless as an oil-pressure warning light, too.)

If you’re certain that you have a good bulb and the light still doesn’t come on, the next thing to do is to check the wiring. Disconnect the D+ wire from the back of the alternator (it’s best to disconnect the battery first so that you don’t short anything to ground), then connect a multimeter between the end of the D+ wire and ground. Crack the key and measure the voltage. If you’re not seeing battery voltage on the D+ wire, there is likely a wiring problem in the voltage supply to the alternator light itself.

If there is battery voltage on D+, temporarily attach the terminal on the end of the D+ wire to chassis ground using a wire with alligator clips at both ends. Then crack the key again. If the light comes on now, it’s likely that the alternator isn’t properly grounded. Check the battery ground cable and the connections on both ends of the alternator ground strap.

As I said, the alternator light has to be working to provide the excitation current. However, there’s a very cool trick you can do if the light isn’t working. You can supply your own excitation current. Run a new wire from the D+ post on the back of the alternator and touch it briefly to the positive battery terminal (or to the large B+ terminal on the alternator, which goes to the battery) to supply the excitation current to jolt the alternator into operation. Obviously, you need to be very careful not to short anything to ground while you’re doing this.

Anyway, let’s get back to poor Kugel. The next day, I went back out to Fitchburg with a brand-new battery and a copy of the alternator chapter of my own book—did you know that you can read about my new book at the end of this column? You did? Well, then.—in tow to help make sure that I didn’t get all this stuff wrong. I found that, indeed, the alternator light was not coming on when I cracked the key.

I swapped bulbs but it made no difference.

I checked the D+ wire and found that it did not have voltage present. So I tried the alternate-exciter trick. I disconnected the battery, connected a wire to D+ on the back of the alternator, ran this wire up near the battery, zip-tied it in place so that it wouldn’t flop around, re-connected the battery, and started the car. I checked the battery voltage with a multimeter: It was around 12.6V. I then touched the exciter wire briefly to the positive battery terminal, took it away, and re-checked the battery voltage. I smiled when I saw that the battery voltage was now about 14 volts; this meant that the alternator was now charging the battery, and would continue to do so until I shut the car off.

I’ll still need to troubleshoot why voltage isn’t reaching the warning light, but the important thing is that I now know that the alternator itself is fine. This was especially valuable information to me, because on a tii, the alternator is down low, below the battery tray; it’s a bear to remove and re-install. The only way I know to do it requires taking the sway bar out. This is not the end of the world, but it’s a bit more than I’d prefer to tangle with in the storage space in Fitchburg, without my tools, jack stands, electricity, and lighting. Now I know that I can bootstrap the alternator manually and drive the car home. Neat trick, huh?

And hey—I learned that my new book is actually useful. You can read about it down—oh, never mind.

Next week: With any luck, we’ll get back to Otto’s poor separated head.—Rob Siegel